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如下代码示例主要是演示多线程场景下SimpleDataFomate类(SimpleDataFormate类是非线程安全的)的使用!,通过该例子去理解ThreadLocal的工作原理
import java.text.SimpleDateFormat;public class Main { public static void main(String[] args) { //使用lambda表达式的语法糖 new Thread( ()-> runTest(1)).start(); new Thread( ()-> runTest(1)).start(); new Thread( ()-> runTest(2)).start(); new Thread( ()-> runTest(2)).start(); } public static void runTest(int mode) { for (int i = 0; i < 2; i++) { try { long times = System.currentTimeMillis(); System.out.println(String.format("当前在[tid=%s]线程,格式转化输出 %s", Thread.currentThread().getId(), mode == 1 ? TimeFormatUtils.formatTimeWithHHmmPattern(times) : TimeFormatUtils.formatTimeWithHHmmssPattern(times))); Thread.sleep(1000 * 3); Thread.sleep(1000 * 5); } catch (InterruptedException e) { e.printStackTrace(); } } }}class TimeFormatUtils { /** * 全局变量,整个运行时存在,在加载TimeFormatUtils类时初始化,sDataFormatThreadLocal 是一个存放SimpleDateFormat实例的容器 * 每个线程都存放自己的SimpleDateFormat实例 */ private static ThreadLocalsDataFormatThreadLocal = new ThreadLocal<>(); /** * 全局变量,整个运行时存在,在加载TimeFormatUtils类时初始化,sDataFormatThreadLoca2l 是一个存放SimpleDateFormat实例的容器 * 每个线程都存放自己的SimpleDateFormat实例 */ private static ThreadLocal sDataFormatThreadLocal2 = new ThreadLocal<>(); //同理可以创建多个ThreadLocal 的实例,并存放T实例(各个线程有各自的T实例) public static SimpleDateFormat getFormatterWithHHmm() { /** * 这里获取当前线程的SimpleDateFormat的实例,如果没有创建一个新的实例,并存储在当前线程的ThreadLocalMap中 */ SimpleDateFormat format = sDataFormatThreadLocal.get(); if (format == null) { format = new SimpleDateFormat("HH-mm"); log(String.format("没有对应的SimpleDateFormat实例(HH:mm), 创建实例之,对象地址为 = %s, 对象toString() = %s", System.identityHashCode(format), format.toString())); sDataFormatThreadLocal.set(format); } else { log(String.format("已经有对应的SimpleDateFormat实例(HH:mm), 对象地址为 = %s",System.identityHashCode(format))); } return format; } public static void log(String info) { System.out.println(String.format("当前在[tid=%s]线程,%s", Thread.currentThread().getId(), info)); } public static String formatTimeWithHHmmPattern(long times) { return getFormatterWithHHmm().format(times); } public static SimpleDateFormat getFormatterWithHHmmss() { /** * 这里获取当前线程的SimpleDateFormat的实例,如果没有创建一个新的实例,并存储在当前线程的ThreadLocalMap中 */ SimpleDateFormat format = sDataFormatThreadLocal2.get(); if (format == null) { format = new SimpleDateFormat("HH:mm:ss"); log(String.format("没有对应的SimpleDateFormat实例, 创建实例(hh:mm:ss)之,对象地址为 = %s, 对象toString() = %s", System.identityHashCode(format), format.toString())); sDataFormatThreadLocal2.set(format); } log(String.format("已经有对应的SimpleDateFormat实例(hh:mm:ss), 对象地址为 = %s", System.identityHashCode(format))); return format; } public static String formatTimeWithHHmmssPattern(long times) { return getFormatterWithHHmmss().format(times); }}
/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/bin/java "-javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=55068:/Applications/IntelliJ IDEA CE.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/lib/tools.jar:/Users/luogw/temp/ThreadLocalDemo/out/production/ThreadLocalDemo Main当前在[tid=11]线程,没有对应的SimpleDateFormat实例(HH:mm), 创建实例之,对象地址为 = 709397301, 对象toString() = java.text.SimpleDateFormat@4180d8d当前在[tid=14]线程,没有对应的SimpleDateFormat实例, 创建实例(hh:mm:ss)之,对象地址为 = 2004765498, 对象toString() = java.text.SimpleDateFormat@8140d380当前在[tid=13]线程,没有对应的SimpleDateFormat实例, 创建实例(hh:mm:ss)之,对象地址为 = 1557412645, 对象toString() = java.text.SimpleDateFormat@8140d380当前在[tid=12]线程,没有对应的SimpleDateFormat实例(HH:mm), 创建实例之,对象地址为 = 1732199956, 对象toString() = java.text.SimpleDateFormat@4180d8d当前在[tid=13]线程,已经有对应的SimpleDateFormat实例(hh:mm:ss), 对象地址为 = 1557412645当前在[tid=14]线程,已经有对应的SimpleDateFormat实例(hh:mm:ss), 对象地址为 = 2004765498当前在[tid=13]线程,格式转化输出 19:24:34当前在[tid=12]线程,格式转化输出 19-24当前在[tid=14]线程,格式转化输出 19:24:34当前在[tid=11]线程,格式转化输出 19-24当前在[tid=13]线程,已经有对应的SimpleDateFormat实例(hh:mm:ss), 对象地址为 = 1557412645当前在[tid=12]线程,已经有对应的SimpleDateFormat实例(HH:mm), 对象地址为 = 1732199956当前在[tid=13]线程,格式转化输出 19:24:42当前在[tid=12]线程,格式转化输出 19-24当前在[tid=14]线程,已经有对应的SimpleDateFormat实例(hh:mm:ss), 对象地址为 = 2004765498当前在[tid=11]线程,已经有对应的SimpleDateFormat实例(HH:mm), 对象地址为 = 709397301当前在[tid=14]线程,格式转化输出 19:24:42当前在[tid=11]线程,格式转化输出 19-24Process finished with exit code 0
如示例代码从数据结构与代码行为两部分说明:
public T get() { //获取当前线程 Thread t = Thread.currentThread(); //获取线程的map,key就是当前ThreadLocal对象,value就是ThreadLoal存放的某个类的实例,这个示例代码是SimpleDataFormat实例 ThreadLocalMap map = getMap(t); if (map != null) { //于ThreadLocal实例自己为key,去查找对应(保存在自己身上的)SimpleDataFormat实例 ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } //线程第一次运行到这里时,map是没有创建的,所以setInitalValue除了初始化线程对应的ThrealdLcaoMap后,还需要初始化当前ThrealLocal对象对应的SimpleDataFormat实例 return setInitialValue(); }
setInitialValue的代码如下
private T setInitialValue() { //初始化当前ThrealLocal对象对应的SimpleDataFormat实例 T value = initialValue(); Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); return value; }
接下来我们来看看,ThrealLocal对象保存数据(保存基对应的SimpleDataFormat)的逻辑,关键是ThreadLocal的set方法
/** * Sets the current thread's copy of this thread-local variable * to the specified value. Most subclasses will have no need to * override this method, relying solely on the {@link #initialValue} * method to set the values of thread-locals. * * @param value the value to be stored in the current thread's copy of * this thread-local. */ public void set(T value) { //获取当前线程 Thread t = Thread.currentThread(); //获取当前线程的ThrealLocalMap ThreadLocalMap map = getMap(t); if (map != null) //把数据保放在map中,即key为ThreadLocal自己,value为其对应的simpleDataFormate实例 map.set(this, value); else createMap(t, value);//线程第一次调用这里是,map是空的,所以需要创建map在存放数据 }
打印SimpleDataFormat对象地址使用System.identityHashCode方法,是因为SimpleDataFormat重裁了hashCode方法,然后里边的实现是调用了字符串(格式化模板参数)的hast code,所以如下的代码输出日志中我们看到不同线程创建的同一格式化模板参数SimpleDataFormat的“地址”是一样的(仅看toString的输出时)
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